Incorporation of Fused Tetrathiafulvalenes (TTFs) into Polythiophene Architectures:  Varying the Electroactive Dominance of the TTF Species in Hybrid Systems

Abstract

A novel polythienylenevinylene (PTV) and two new polythiophenes (PTs), featuring fused tetrathiafulvalene (TTF) units, have been prepared and characterized by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroelectrochemistry. All polymers undergo two sequential, reversible oxidation processes in solution. Structures in which the TTF species is directly linked to the polymer backbone (2 and 4) display redox behavior which is dictated by the fulvalene system. Once the TTF is spatially removed from the polymer chain by a nonconjugated link (polymer 3), the electroactivity of both TTF and polythiophene moieties can be detected. Computational studies confirm the delocalization of charge over both electroactive centers (TTF and PT) and the existence of a triplet dication intermediate. PTV 4 has a low band gap (1.44 eV), is soluble in common organic solvents, and is stable under ambient conditions. Organic solar cells of polymer 4:[6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) have been fabricated. Under illumination, a photovoltaic effect is observed with a power conversion efficiency of 0.13% under AM1.5 solar simulated light. The onset of photocurrent at 850 nm is consistent with the onset of the pi-pi absorption band of the polymer. Remarkably, UV-vis spectroelectrochemistry of polymer 4 reveals that the conjugated polymer chain remains unchanged during the oxidation of the polymer.

Fused H-shaped tetrathiafulvalene–oligothiophenes as charge transport materials for OFETs and OPVs

“…[1][2][3] In recent years, this has 20 included the use of TTF and its derivatives as components in field-effect transistors, where several examples of materials demonstrating high mobilities have been reported. [4][5][6][7] Similarly, poly-and oligothiophenes have been shown to be viable semiconducting materials in a variety of organic electronic 25 devices, including light emitting diodes, 8,9 organic solar cells [10][11][12] and organic field effect transistors (OFETs).…”

“…Thus, it has been shown that the role of the polymer/oligomer is crucial in determining whether the fulvalene unit can either dominate the electroactivity of the molecule, or participate in a hybrid redox state. 20 The electrochemical studies showed that, due to the TTF system being fused to the conjugated 40 chain through only one of the 1,3-dithiole rings, the second ring was free to act independently, providing the signature oxidation of the second dithiole moiety.…”

“…Treatment with n-butyllithium and carbonylation 20 with DMF resulted in the formation of aldehyde 10 in 78% yield. With the requisite aldehydes 6 and 10 in hand, completion of the syntheses of 1 and 2 followed that of septithiophene 3.…”

“…In contrast, the absorption maxima of 2 and 3 are blue-shifted by 5 nm and 6 nm respectively, when compared to their corresponding half-units 17 and 18 (λmax = 431 nm for 2, vs 436 nm for 17, and λmax = 461 nm for 3, vs 467 nm for 18). In the case of quinquithiophene-TTF 2, this can be explained by 10 considering the disruption of planarity in the thiophene chains, caused by the close proximity of the 3-hexyl chains on the terminal thiophenes, which would point inwards, towards each other in an all-anti 20 conformation. In the larger septithiophene analogue 3, the 3-hexyl chains are significantly further apart, so it is unlikely that the observed blue-shift for absorbance is caused by steric repulsion between the hexyl chains.…”

“…A single reversible oxidation at a 10 half-wave potential of +0.24 V, followed by three quasireversible oxidations at higher potentials of +0.51 V, +0.74 V and +0.92 V. Analogously to compounds 1 and 3, quinquithiophene-TTF 2 exhibits a single, irreversible reduction peak at -2.19 V. Since the first and second oxidation half-wave potentials for 17 15 are +0.50 V and +0.62 V (see supporting information), one can assume that the first and second oxidation potentials for 2 are due to the TTF and quinquithiophene species, respectively. The second oxidation potential for 1, which is assigned to the TTF unit, has a half-wave value of +0.81 V; therefore the third 20 oxidation process for 2, with a half-wave potential of +0.74 V, is likely to arise from the electroactivity of the quinquithiophene chains. The fourth oxidation peak of 2 (E1/2 = +0.93 V) is due to the second oxidation of the TTF unit.…”